Arrangement for measuring at frequencies actually used for signalling, the condition of a receiving antenna positioned apart from other base station equipment at a base station of a radio system

ABSTRACT

An arrangement for measuring the condition of a receiver antenna at a base station, utilizing an antenna amplifier positioned in connection with an antenna apart from other base station equipments, e.g., at a mast. A measuring signal is generated in connection with the other base station equipments at a frequency outside the frequency band used for radio traffic and applied through an antenna line to the antenna amplifier, in which the measuring signal is converted to the actual measuring frequency within the frequency band intended for the radio traffic.

BACKGROUND OF THE INVENTION

The invention relates to an arrangement for measuring the condition of areceiver antenna at a base station of a system, the arrangementcomprising a receiver antenna positioned apart from other base stationequipments, e.g., at a mast; an amplifying means positioned inconnection with the antenna for amplifying an antenna signal; a meansfor generating a radio frequency measuring signal; a first directionalcoupler means for applying the measuring signal to an antenna linetowards the antenna between the amplifying means and the antenna; asecond directional coupler means for applying the measuring signal tothe antenna line towards the receiver between the amplifying means andthe antenna; switching means for switching the radio frequency measuringsignal from the generating means alternately to the first and the seconddirectional coupler means; means for measuring the strength of ameasuring signal component sent towards the antenna and reflected backfrom the antenna and the strength of the measuring signal sent directlytowards the receiver.

A substantial part of radio systems, for instance cellular radiotelephone systems and base stations thereof, are receiver andtransmitter antennas, the condition of which influences the quality ofconnections. A control of the condition of the antennas can be carriedout e.g., by measuring the SWR (Standing Wave Ratio) of the antennas,i.e., electrical matching of the antennas to the remaining part of thereceiver and transmitter system.

By means of present methods, the measurement is performed by supplyingpower through the antenna line to the receiver antenna and by measuringthe power reflected back from the antenna along the antenna line bymeans of a broadband power meter. Due to the broadband power meter, thepower used for the measurement must be high in order that themeasurement will not be sensitive to interference, which means thatsignals received by the antenna do not disturb the measurement. Usinghigh power causes intermodulation distortion in the receiver. Theseproblems can be avoided by using a duplex filter separating from eachother the measuring signal and the signals intended to be received.However, the duplex filter must be installed before the receiver parts,which weakens the sensitivity of the receiver, because the filter causeslosses in the antenna signal.

A further solution is known, avoiding both above-mentioned problems byusing a measuring frequency outside the receiver band and a narrow-bandpower measurement adapted to this frequency.

European Patent Application 0 261 828 discloses an analyzer of microwaverange, measuring a signal supplied directly from a measuring source aswell as a signal reflected from the microwave network to be analyzed,for a vector-based relative power measurement. At measurement ofreflected power, a sample signal taken of the output power of ameasuring generator is brought to a detector along a path different fromthe path of a sample signal taken of the power reflected from themicrowave network to be analyzed. Thus, the relative measurement of thereference cited would be somewhat insensitive to variations in theoutput level of the measuring generator, but since the signals to becompared with each other use different paths to the detector, themeasurement does not automatically consider non-idealities or changes inthe properties of components present on signal paths. The analyzer mustbe calibrated for the measurement of reflected power by measuring aknown microwave standard at first with the analyzer and by usingthus-obtained calibration values for later measurements. For measuringthe condition of an antenna, this would mean that the power supply ofthe antenna should be connected to some microwave standard instead ofthe antenna for the calibration.

This problem has been solved in the Finnish Patent Application 904085 insuch a way that, in addition to the signal reflected from the antenna,also the strength of the measuring signal sent directly towards thereceiver is measured separately. By this second measurement is obtaineda reference value for the strength of the measuring signal, which valueconsiders at the measuring moment the transmission power of a measuringsignal transmitter and the properties of the components present on thesignal path, such as amplifiers and branching elements, and with whichvalue the strength of a measuring signal component reflected from theantenna is compared. By a relative measurement of this kind, it ispossible to eliminate the influence of the components present in themeasuring circuit and on the measuring signal path between theproperties of individual components, or the influence of temporalchanges in the properties of an individual component on the accuracy ofmeasurement. Manual checkings and calibrations during installation andoperation can also be reduced and simplified substantially or they caneven be entirely avoided. The measuring signal is preferably anarrow-band signal, the frequency of which is outside the frequency bandallocated for traffic, due to which the measurement does not disturb theactual radio traffic, but, on the other hand, the measurement is notperformed at actually used frequencies either, and for this reason, anactual standing wave ratio SWR will not be obtained.

New digital TDMA-type (TDMA=Time Division Multiple Access) radio systemshave time-division signalling including several, typically 8, time slotsat one frequency. One TDMA system is the pan-European mobile telephonesystem GSM. The GSM Specification 12.11, 3.1.0 B 05 "Receiver AntennaFault" states requirements for the control of the condition of theantenna.

With the antenna positioned at the mast and provided with a mastpreamplifier, the condition of the antenna cannot be measured in anormal way, because the preamplifier is located on the antenna linebetween the feed point of the measuring signal and the antenna and themeasuring signal should pass through this preamplifier in reversedirection. An attenuation of the amplifier in reverse direction istypically 40 dB. The accuracy of measurement is influenced byattenuations of even a few decibels, such as a receiver filter, which isalso positioned at the mast. There is thus a need of providing the mastwith a switching point for the measuring signal between the mastpreamplifier and the antenna.

SUMMARY OF THE INVENTION

The object of the invention is to implement a measurement of thecondition of a receiver antenna provided with a mast preamplifier at aTDMA base station at actually used frequencies.

This is achieved by means of an arrangement of the type presented in theintroduction, the arrangement being according to the inventioncharacterized in that the measuring signal to be switched to thedirectional coupler means has a first frequency on a frequency bandallocated for the radio traffic of the radio system, and that thegenerating means are positioned in connection with the other basestation equipments and comprise means for sending the measuring signalthrough the antenna line to the switching means at a second frequencyoutside the frequency band allocated for the radio traffic of the radiosystem, and that the switching means comprise a means connected to theantenna line between the amplifying means and the other base stationequipments for the reception of the measuring signal at the secondfrequency and mixer means for converting the measuring signal from thesecond frequency to the first frequency before the measuring signal isswitched to the directional coupler means.

In the invention, the measuring signal is generated in connection withthe other base station equipments and applied outside the frequency bandused for the traffic through the antenna line to the preamplifier, inwhich the measuring signal is converted to the actual measuringfrequency within the frequency band intended for the traffic.

In a preferred embodiment of the invention, the radio system is a TDMAsystem and the measurement is performed in a time slot allocated for aradio test loop, whereby frequencies actually used for the traffic canbe utilized. In one embodiment of the invention, a radio frequencymeasuring signal is generated at transmission frequency in a transmitterand looped through the radio test loop at reception frequency to thereceiver side, thereby avoiding a separate measuring signal generator.By means of the invention, the signal in its original form is obtainedto a switching point at the mast without interfering the received signalfrom the antenna within the actual reception band.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be explained in more detail bymeans of illustrative embodiments with reference to the attacheddrawing, in which

FIG. 1 shows a block diagram of receiver equipments of a base stationaccording to the invention,

FIG. 2 shows a block diagram of a ground unit 8 of FIG. 1,

FIG. 3 shows a block diagram of a mast unit 15 of FIG. 1.

DETAILED DESCRIPTION

The invention is suitable for being used especially in a fullduplex-type transceiver of a base station of TDMA radio systems.

In FIG. 1, a TDMA transceiver comprises at least two, preferably 4,pairs of transceivers 1A, 10A and 1B, 10B, respectively, each pairconstituting one full duplex connection. The outputs of the transmitterunits 1A and 1B are connected by means of a combining element 2(combiner) to a common antenna line 3 and to a common transmitterantenna 4. A receiver antenna 6 positioned apart from other base stationequipments, e.g., at a mast, is connected via an associated preamplifierunit 15 (hereafter called a mast unit) and an antenna line 7 to abranching element 9, which divides the received signal to the receiverunits 10A and 10B. In this connection, transmitter and receiver unitsprimarily signify radio parts of a transceiver. Each transmitter andreceiver unit has an individual transmission or reception frequency,whereby the transmission and reception frequencies of the units, such as1A and 10A, constituting a full duplex pair, are at a distance of aduplex spacing, e.g., 45 MHz, from each other.

A receiver Rx receives from the antenna a TDMA signal, each frequency ofwhich comprises several, typically 8, time slots. The signal hasadditionally a frame structure having e.g., eight successive time slotsconstituting a frame. Further, frames can form multiframes (e.g., 26 or52 frames) and multiframes hyperframes. One TDMA system is thepan-European mobile telephone system GSM. Time slots are mainly used fortransferring control and traffic channels.

Transceivers often use a radio test loop, in which a radio frequencytest signal generated by the transmitter part is led in a correct mannerback to the receiver part of the same transceiver for reception andanalysis. Such a radio test loop of a transceiver is described e.g., inthe Finnish Patent 85080 included herein as a reference.

In the GSM system, for instance, at least one time slot of a frame,multiframe or hyperframe is, however, allocated for an establishment ofa radio test loop from the transmitter to the receiver.

A radio test unit 11 is connected between the antenna lines 5 and 7. Forthis purpose, the antenna line 5 comprises a branching element 3, whichbranches-off a portion of a transmission RF signal to an input 12 of theradio test unit 11. The radio test unit 11 converts the transmissionfrequency signal into a reception RF signal. An output 13 of the radiotest unit 11 is connected to the antenna line 7 by means of a switchingunit 8, hereafter called a ground unit. The radio test loop is normallyintended for testing the RF parts of the base station, except thereceiver antenna and the receiver antenna cable.

In a preferred embodiment of the invention, the condition of thereceiver antenna 6 is measured in a time slot allocated for the radiotest loop. In the embodiment of FIG. 1, a radio test signal loopedduring a radio test time slot from the transmitter part to the receiverantenna line is used as a measuring signal and sent on a radio channel(at a frequency) used for normal traffic of the radio system.

FIG. 2 shows the sub-unit 8 of FIG. 1 in greater detail. A measuringsignal at reception frequency fr obtained from the radio test unit 11via coupling condenser C is mixed in a mixer 20 to a frequency faoutside the reception band by means of a local oscillator signal LOobtained from a local oscillator 22. For instance, when the signal 13has a frequency of 890 to 915 MHz and the signal LO a frequency of 120MHz, the measuring signal can be converted to a frequency higher orlower than 120 MHz, e.g., to a range of 770 to 795 MHz. The upperharmonic of the mixing result is filtered off by a passband filter 21and the down-converted measuring signal is switched by a directionalcoupler 24 to the antenna cable 7 towards the antenna 7.Correspondingly, oscillators 25 and 26 generate control signals havingfrequencies f1 and f2. The control signals are connected to the antennacable 7 by a coil L2. A logic unit 23 controls the operation of theoscillators 22, 25 and 26, switching them on and off according to thecontrol of line 14 B from the maintenance unit (OMU) 14 and the DC levelobtained on line 13 and via coil L1 from the test unit 11. When theoscillator 22 is switched on, the oscillator 26 is also in operation andthe frequency f2 is transmitted. When the oscillator 22 is switched off,the frequency f2 is not transmitted either. As will be described laterin more detail, this arrangement makes it possible that both anoscillator 39 of the mast unit 15 and the oscillator 22 of the groundunit 8 are switched on only during the measurement of the antenna.Possible interferences caused by the oscillators in conventional radiotraffic are thus prevented. The ground unit 8 obtains an information ofthe moment when the antenna is measured from the OMU 14 via line 14B. Onthe other hand, the DC level of the signal 13 indicates the switchingdirection of the measuring signal, i.e. towards the receiver or theantenna. This information of direction is converted into the frequencyf1. When f1 is transmitted, the switching direction is towards theantenna. When f1 is lacking, the switching direction is towards thereceiver.

The mast unit 15 in connection with the antenna preamplifier of the mastis illustrated in more detail in FIG. 3. The unit 15 comprisesconventionally a receiver filter 30 and an amplifier unit secured on hotstand-by principle, consisting of a branching element 31, parallelamplifier branches 32 and 33 and a combining element 34. The actualmeasuring equipment comprises a directional coupler 35, which takes fromthe antenna cable 7 a measuring signal sent by the ground unit 8 at thefrequency fa, the other frequencies, such as the actual receptionfrequencies, around the frequency fa being filtered off by means of apassband filter 36 (band width e.g., 770 to 795 MHz). The filteredmeasuring signal is amplified by an amplifier 37 and then converted in amixer 38 back to the reception frequency fr by the local oscillatorsignal LO obtained from the oscillator 39. The signal LO has the samefrequency as the signal LO of the ground unit 8. Frequencies outside thereceiver band are eliminated from the mixing result by means of apassband filter 40 and the filtered signal is brought to a switch S4. Aswitch 41 switches the measuring signal selectively and alternatingly todirectional couplers 42 and 43 connecting the measuring signal to theantenna cable 7 between the mast unit 15 and the antenna 6. Thedirectional coupler 42 supplies the measuring signal towards thepreamplifier and the receiver and the directional coupler 43 towards theantenna 6. In this manner, a measuring signal identical with theoriginal signal 13 can be switched to the antenna line 7 between theantenna 6 and the unit 15 before the receiver filter 30 withoutinterferring the actual receiver signal going in another direction.

The switch 41 and the oscillator 39 are controlled by a control logic 44according to control signals f1 and f2 received from the antenna line.The control signals are separated from the antenna cable 7 coming fromground unit 8 by the coil L and filtered by means of passband filters 45and 46. The filtered control signals are detected in the control logic44. Upon detecting the frequency f1, the control logic 44 controls theswitch 41 to position I, in which the measuring signal is switched tothe directional coupler 43. If frequency f1 does not occur, the controllogic 44 controls the switch 41 to position II, in which the measuringsignal is switched to the directional coupler 42. Upon detecting thefrequency f2, the control logic 44 switches on the oscillator 39. Whenfrequency f2 does not occur, the control logic 44 switches off theoscillator 39. By means of this arrangement, the oscillator 39 of themast unit 15 as well as the oscillator 22 of the ground unit 8 areswitched on only during the measurement of the antenna. Possibleinterferences caused by the oscillators in normal radio traffic are thusprevented. The ground unit 8 receives an information of the moment whenthe antenna is measured from the OMU 14 through line 14B.

The measurement takes place at normal reception frequencies of the radiosystem during normal operation of the receiver Rx. Test operations arecontrolled by the operation and maintenance unit 14 of the base station,which unit commands the test unit 11 through control line 14A to form atest loop within a predetermined time slot and informs whether themeasuring signal is switched towards the antenna or towards thereceiver. In the preferred embodiment of the invention, the test unit 11then controls the ground unit 8 by the DC level of the output 13.Direction control can also be given directly from the OMU 14 to thesub-unit 8 through control line 14D. Outside the test time slot, themeasuring signal is disconnected from the output 13 by means of adecoupling switch within the unit 11.

Moreover, the OMU 14 acts as a measuring unit, which receives a signalfrom the branching element 9 and measures the strength of the signal.Alternatively, the receivers Rx may contain a measuring equipment, themeasuring result of which is received by the OMU 14 through line 14C.

The measuring procedure is as follows. The quantity to be measured isthe standing wave ratio SWR of the antenna.

1) The OMU 14 gives the test unit 11 a normal radio test loop command,in consequence of which a measuring signal is looped in a test time slotto the ground unit 8. The OMU 14 also informs the ground unit 8 of theantenna measuring transaction, due to which the oscillator 22 starts andthe measuring signal is converted to the frequency fa and transmitted toan upper unit 15, in which it is reconverted to the frequency fr andswitched, depending on the direction information, e.g. to thedirectional coupler 42, i.e. directly towards the receiver. The OMU 14measures the strength of the measuring signal and uses an RSSI value ofthe strength of the received signal, formed from the measurement result,as a reference value REFSIGN.

2) The OMU 14 gives the test unit 11 a new radio test loop command todirect the measuring signal towards the antenna 6, however. The switch41 switches the measuring signal to the directional coupler 43. The OMU14 measures the strength of a signal component reflected from theantenna 6 and uses the RSSI value formed from the measurement result asa measured value ANTSIGN.

3) The OMU 14 calculates the SWR of the antenna from these two valuesANTSIGN and REFSIGN.

4) The OMU 14 compares the calculated SWR value with stored alarmlimits. If the calculated value is higher/lower than the alarm limits,the alarm is given. The data base of the OMU 14 contains a calibratedSWR or SRRI value for each used frequency, preferably also for eachantenna sector and both for a normal antenna and a diversity antenna e.During the setup of the base station or when the antenna or the antennacable is changed, the measurements 1, 2 and 3 of each frequency, sectorand antenna are gone through and the SWR values obtained as a result arestored as calibrated values in the data base of the OMU.

The drawing figures and the description provided above are only intendedto illustrate the present invention. As to the details, the arrangementaccording to the present invention may vary within the scope of theappended claims.

I claim:
 1. An arrangement for measuring the condition of a receiverantenna at a base station of a radio system, comprising:said receiverantenna being positioned remote from other base station equipments; anamplifying means positioned in connection with said antenna foramplifying an antenna signal; a means for generating a radio frequencymeasuring signal; a first directional coupler means for applying saidmeasuring signal to an antenna line towards said antenna between saidamplifying means and said antenna; a second directional coupler meansfor applying the measuring signal to said antenna line towards saidreceiver between said amplifying means and said antenna; switching meansfor switching the radio frequency measuring signal from said generatingmeans alternately to said first and second directional coupler means;means for measuring the strength of a measuring signal component senttowards said antenna and reflected back from said antenna and thestrength of the measuring signal sent directly towards said receiver;the measuring signal to be switched to the directional coupler meanshaving a first frequency on a frequency band allocated for radio trafficof said radio system; said generating means being positioned inconnection with said other base station equipments and comprising meansfor sending the measuring signal through said antenna line to saidswitching means at a second frequency outside said frequency bandallocated for radio traffic of said radio system; said switching meanscomprising a means connected to said antenna line between saidamplifying means and said other base station equipments for thereception of said measuring signal at said second frequency and mixermeans for converting said measuring signal from said second frequency tosaid first frequency before said measuring signal is switched to one ofsaid first and second directional coupler means.
 2. An arrangementaccording to claim 1, wherein:said radio system is a TDMA system; andtransmission of a measuring signal at the first frequency is permittedonly in a TDMA time slot of said base station intended for anestablishment of a radio test loop.
 3. An arrangement according to claim1, wherein:said generating means comprise a radio test loop for loopingsaid measuring signal from a transmitter of said base station at saidfirst frequency.
 4. An arrangement according to claim 1, wherein:saidgenerating means comprise a measuring signal generator for generatingsaid measuring signal at said first frequency.
 5. An arrangementaccording to claim 1, wherein:said generating means comprise mixer meansfor converting said measuring signal from said first frequency to saidsecond frequency before said measuring signal is sent through saidantenna line to said switching means.
 6. An arrangement according toclaim 1, wherein;said generating means comprise means for sendingswitching state information through said antenna line to said switchingmeans; and said switching means is responsive to said switching stateinformation for switching said measuring signal selectively to saidfirst and second directional coupler means.
 7. An arrangement accordingto claim 6, wherein:said means for sending switching informationcomprise generator means for generating a control signal at a thirdfrequency, when said measuring signal is switched to said firstdirectional coupler means.
 8. An arrangement according to claim 7,wherein:said switching means comprise a change-over switch means,detector means for detection of said control signal, and control meansresponsive to said detector means for bringing said change-over switchmeans to a first position, in which said measuring signal is switched tosaid first directional coupler means when said detector means detectsaid control signal at said third frequency, and to a second position,in which said measuring signal is switched to said second directionalcoupler means when said detector means does not detect said controlsignal at said third frequency.
 9. An arrangement according to claim 1,wherein:said generating means comprise means for sending operating stateinformation through said antenna line to said switching means; and saidswitching means are responsive to said operating state information forswitching on and off a local oscillator of said mixer means.
 10. Anarrangement according to claim 1, wherein:said base station includes amast; and said receiving antenna is positioned on said mast.